Originally Posted by
Equinox
So I think I've figured out how a barrel adjuster works. It controls the length of the housing. And I also think I understand how turning it in a particular direction effects the lenghth of the housing. I just don't understand the relationship of housing length to cable tension. I seem to have both answers here. Tighten the cable by lengthening the housing. Tighten the cable by shortening the cable.
You tighten the cable by lengthening the housing, like I explained. If the effective length of the housing increases, more of the cable will be inside it and less will be sticking out. Less cable sticking out pulls the derailleur slightly further in whichever direction shortening the cable moves it.